Contraintes de germination et diagnostic moléculaire des champignons associés aux maladies chez Ricinodendron heudelotii au Cameroun

Cette étude avait pour but d’inventorier la flore fongique associée aux symptômes observés sur les organes de Ricinodendron heudelotii (Baill.) Pierre ex Heckel au Cameroun dans une perspective d’agroforesterie. De manière spécifique, il s’est agi de : 1) dégager les atouts et les contraintes agroforestières de R. heudelotii au Cameroun en identifiant des thématiques de recherche à venir pour cette essence; 2) identifier les contraintes de germination de cette essence et d’y apporter des solutions; 3) caractériser, du point de vue phénotypique, quelques champignons présumés pathogènes chez R. heudelotii; 4) vérifier le pouvoir pathogène (postulats de Koch) de ces champignons présumés pathogènes et 5) tester l’efficacité de quatre fongicides de synthèse contre les maladies foliaires des plantules de R. heudelotii en pépinières. La dormance tégumentaire, la durée de conservation des graines et les maladies sont les contraintes de germination chez R. heudelotii. Les recherches menées dans les bases de données publiques en utilisant l’algorithme BLAST (Basic Local Alignment Search Tool) et les séquences du gène de l’ADNr cloné (PCR) des 350 isolats collectés ont montré que plusieurs champignons d’importance phytopathologique colonisent les organes infectés de R. heudelotii au Cameroun. Le milieu de culture et la température d’incubation ont affecté les caractéristiques phénotypiques (pigmentation et vitesse de croissance du mycélium, concentration de conidies) des champignons présumés pathogènes chez R. heudelotii à savoir Pestalotiopsis microspora, Lasiodiplodia theobromae et trois individus du taxon Fusarium oxysporum (F. oxysporum ZB049F, F. oxysporum ZB164T et F. oxysporum ZH121F). Ces cinq champignons se sont révélés pathogènes sur les organes détachés et les plantules de R. heudelotii en laboratoire; leur agressivité chez l’hôte a varié d’une espèce fongique à l’autre. Les préparations commerciales de fongicides Plantomil 72WP (60% d’oxyde de cuivre et 12% de métalaxyl) et Nordox 75WG (86% d’oxyde de cuivre) ont permis de lutter efficacement contre les maladies foliaires dues à P. microspora et L. theobromae. Compte tenu de la grande diversité des espèces dans le taxon F. oxysporum et de l’importance phytopathologique de F. solani, l’identification moléculaire plus poussée et complète des individus de ces espèces isolés chez R. heudelotii est à envisager.The work described in this thesis was conducted in order to identify the fungal species that are associated with infected organs of Ricinodendron heudelotii (Baill.) Pierre ex Heckel in Cameroon in the perspective of agroforestry. Specifically, objectives were to: 1) identify the advantages and constraints of R. heudelotii for agroforestry in Cameroon and suggest some future research topics for this species; 2) identify germination constraints of R. heudelotii and provide solutions; 3) characterize morphocultural parameters of some suspected pathogenic fungi of R. heudelotii; 4) verify Koch’s postulates for these suspected pathogens; and 5) test the efficacy of four synthetic fungicides against foliar diseases of R. heudelotii seedlings in nurseries. Seed coat dormancy, duration of storage and diseases were the major factors that affected germination in R. heudelotii. Research in the public databases using the BLAST algorithm (Basic Local Alignment Search Tool) and cloned rDNA sequences of isolated fungi (350 isolates) showed that several fungi of phytopathological interest colonized infected organs of R. heudelotii in Cameroon. Culture medium and temperature affected the phenotypic characteristics (pigmentation of the mycelium, growth rate and conidia concentrations) of suspected pathogenic fungi of R. heudelotii, namely Pestalotiopsis microspora, Lasiodiplodia theobromae and three species of the taxon Fusarium oxysporum (F. oxysporum ZB049F, F. oxysporum ZB164T and F. oxysporum ZH121F). These five fungi were pathogenic on detached organs and seedlings of R. heudelotii under laboratory conditions; their aggressiveness on the host varied from one fungal species to another. Fungicides such as Plantomil 72WP (60% copper oxide and 12% metalaxyl) and Nordox 75WG (86% copper oxide) were effective against foliar diseases caused by P. microspora and L. theobromae. Given the great diversity of species in the taxon F. oxysporum and the phytopathological importance of F. solani, further and complete identification of individuals of those species based on molecular markers should be considered

Genetic divergence in common bean genotypes from the IRAD gene bank: morpho-agronomic characteristics, fungal and bacterial disease resistance, and opportunities for genetic improvement

For successful plant breeding in any crop species, the importance of diversity in the available germplasm population is known and established. Thirty-two common bean (Phaseolus vulgaris) genotypes from the IRAD gene bank in Cameroon were evaluated for divergence in terms of their morpho-agronomic traits, fungal disease resistance, and bacterial disease resistance to assess the opportunity for genetic improvement of the crop. The trait associations were estimated using correlation coefficients and genotypes were classified into groups using cluster and principal component analyses. Seven qualitative and 16 quantitative traits comprising growth, phenological, yield, and disease variables were evaluated in this study. The qualitative markers revealed the degree of polymorphism among the 32 common bean genotypes. The number of phenotypic classes per character (Na) ranged from 2 to 18, with an average of 5.14. The expected gene diversity (He) ranged from 0.37 to 0.93 (mean = 0.56). The number of effective phenotypic classes (Ne) ranged from 1.82 to 14.22, with a mean of 3.85. An extensive range of variation was evident for the majority of traits, highlighting their utility for characterizing common bean germplasm. Many qualitative traits, including seed coat color, seed shape, and seed size, and also some quantitative traits of economic importance including seed yield, were found to be highly variable within the collection, with the MAC55 genotype displaying the highest yield (32.65 g per plant). Four genotypes, namely MAC55, BOA-5-1M6, FEB 192, and Banguem showed resistance to the two main common bean diseases, angular leaf spot and common blight. We detected highly significant correlations among several traits related to yield. A high broad-sense heritability was found for most of the quantitative traits. We carried out two-dimensional principal component analysis and used hierarchical clustering to group the analyzed germplasm according to their phenotypic similitudes. The evidence of agro-morphological diversity in the present collection and the identification of discriminant characters between the available germplasm through the use of PCA analysis have significant implications for establishing breeding schemes in common bean

Monitoring and dietary risk assessment of 81 pesticide residues in 11 local agricultural products from the 3 largest cities of Cameroon

This study monitored 81 pesticides residues in 160 samples of 11 dry agricultural products collected in the 3 largest cities of Cameroon, extracted using QuEChERS method and analysed by liquid chromatography tandem mass spectrometry (LC-MS/MS). Residues of 58 (71.6%) compounds were found in the samples, the most distributed pesticides were Imazalil, Triadimenol and Pyrimethanil, and those with the highest average concentrations were Cymoxanil, Thiamethoxam and Thifensulfuron. Half of the positive pesticides were above their European Union maximum residue limits (MRLs) among which Carbaryl, Carbofuran, Malathion, Metalaxyl and Propoxur are pesticides banned in the country. All the 11 food items contained pesticides, the highest contamination rates (12.8%–5.0%) were found in white pepper, maize, Egusi seeds and groundnuts, while groundnuts, Egusi seeds, maize and soybeans showed the highest residue concentrations (1.46–1.37 mg/kg). Pesticide contamination rates were similar in the 3 sampling cities, but Bafoussam and Yaounde had more samples above the MRLs than Douala. Using the food consumption data for Cameroon from the recent Sub-Saharan Africa Total Diet Study, dietary exposure was calculated and potential health risk of Cameroonian consumers was evaluated. Hazard quotient of Carbofuran in groundnuts was 22% above the safe value, the remaining food items could be considered safe for individual pesticide residues, although Triazophos and Metribuzin in maize were of concern. Groundnuts (0.531) and maize (0.443) showed high hazard index, with 17 highly contributing compounds, but there is no reason to be concerned about cumulative exposure to residues from the food items. While the food items are in general safe to eat, to minimize the increasing human health risk of consumers and ensure approval of Cameroon export produces on international market, this study suggests that authorities must regulate the usage of agrochemicals, strengthen the controls for effective implementation of the pesticide bans and implement strong control of obsolete pesticide stocks in the country

Global patterns in endemicity and vulnerability of soil fungi

Fungi are highly diverse organisms, which provide multiple ecosystem services. However, compared with charismatic animals and plants, the distribution patterns and conservation needs of fungi have been little explored. Here, we examined endemicity patterns, global change vulnerability and conservation priority areas for functional groups of soil fungi based on six global surveys using a high-resolution, long-read metabarcoding approach. We found that the endemicity of all fungi and most functional groups peaks in tropical habitats, including Amazonia, Yucatan, West-Central Africa, Sri Lanka, and New Caledonia, with a negligible island effect compared with plants and animals. We also found that fungi are predominantly vulnerable to drought, heat and land-cover change, particularly in dry tropical regions with high human population density. Fungal conservation areas of highest priority include herbaceous wetlands, tropical forests, and woodlands. We stress that more attention should be focused on the conservation of fungi, especially root symbiotic arbuscular mycorrhizal and ectomycorrhizal fungi in tropical regions as well as unicellular early-diverging groups and macrofungi in general. Given the low overlap between the endemicity of fungi and macroorganisms, but high conservation needs in both groups, detailed analyses on distribution and conservation requirements are warranted for other microorganisms and soil organisms

Global patterns in endemicity and vulnerability of soil fungi

Fungi are highly diverse organisms, which provide multiple ecosystem services. However, compared with charismatic animals and plants, the distribution patterns and conservation needs of fungi have been little explored. Here, we examined endemicity patterns, global change vulnerability and conservation priority areas for functional groups of soil fungi based on six global surveys using a high-resolution, long-read metabarcoding approach. We found that the endemicity of all fungi and most functional groups peaks in tropical habitats, including Amazonia, Yucatan, West-Central Africa, Sri Lanka, and New Caledonia, with a negligible island effect compared with plants and animals. We also found that fungi are predominantly vulnerable to drought, heat and land-cover change, particularly in dry tropical regions with high human population density. Fungal conservation areas of highest priority include herbaceous wetlands, tropical forests, and woodlands. We stress that more attention should be focused on the conservation of fungi, especially root symbiotic arbuscular mycorrhizal and ectomycorrhizal fungi in tropical regions as well as unicellular early-diverging groups and macrofungi in general. Given the low overlap between the endemicity of fungi and macroorganisms, but high conservation needs in both groups, detailed analyses on distribution and conservation requirements are warranted for other microorganisms and soil organisms

Connecting the multiple dimensions of global soil fungal diversity

How the multiple facets of soil fungal diversity vary worldwide remains virtually unknown, hindering the management of this essential species-rich group. By sequencing high-resolution DNA markers in over 4000 topsoil samples from natural and human-altered ecosystems across all continents, we illustrate the distributions and drivers of different levels of taxonomic and phylogenetic diversity of fungi and their ecological groups. We show the impact of precipitation and temperature interactions on local fungal species richness (alpha diversity) across different climates. Our findings reveal how temperature drives fungal compositional turnover (beta diversity) and phylogenetic diversity, linking them with regional species richness (gamma diversity). We integrate fungi into the principles of global biodiversity distribution and present detailed maps for biodiversity conservation and modeling of global ecological processes

Contamination of foods from Cameroon with residues of 20 halogenated pesticides, and health risk of adult human dietary exposure

(1) Background: Halogenated pesticides are abundantly used in Cameroon, but there is no information on the health risk of consumers from exposure to their residues in foods. (2) Methods: Residues of 20 halogenated pesticides were determined in 11 agricultural products collected in the 3 largest cities of Cameroon using QuEChERS extraction and gas chromatography with electron capture detector (GC-ECD), and health risk from dietary exposure was assessed. (3) Results: Organochlorines pesticides aldrin, p,p’-dichlorodiphenyl-trichloroethane (DDT) and β-hexachlorocyclohexane (β-HCH) found in 85.0%, 81.9% and 72.5% of samples, respectively, were the most frequently detected. The highest average concentrations of residues were 1.12, 0.74 and 0.39 mg/kg for methoxychlor, alachlor and β-HCH, respectively, found in chilli pepper. Chili pepper (58.9%), cowpea (56.8%), black beans (56.5%) and kidney beans (54.0%) exhibited the highest residue occurrences. Levels above the European Union maximum residue limits (MRLs) were found for all the 20 pesticides, in 40.1% of the positive analyses, and the food samples contained 14 pesticides banned in Cameroon. Chronic, acute, cumulative and carcinogenic risk assessments revealed that lifetime consumption of maize, black beans, kidney beans, groundnuts and chili pepper contaminated with aldrin, dieldrin, endrin, HCB, heptachlor, o,p’-DDT, p,p’-DDD, p,p’-DDT, p,p’-DDE and β-HCH, could pose health risks. (4) Conclusion: These results show that there is an urgent need of pesticide usage regulation, effective application of pesticide bans and management of obsolete pesticide stocks in Cameroon